During a breaking operation of charging current with a disconnecting switch (DS) in SF6 gas-insulated switchgear (GIS), the occurrence of a fault to ground from the DS contact by a restriking surge was reported to be observed. The flashover voltage in this case, is appreciably lower than the normal withstand voltage to the ground. It has been clarified through high-speed photography with an image converter camera that the flashover is caused by a streamer generated during restriking between the DS contacts. In this respect, with the objective of confirming these phenomena in a simulated power system, two synthetic test circuits were constructed by using 550kV DS's. Verification tests were carried out to confirm the equivalence between these two test circuits. The DS's subjected to test withstood a surge voltage of 2.3pu-the maximum voltage obtained by simulating calculations of various GIS configurations.